Phase determination of persistent heavy rainfall related to 10-30-day intraseasonal oscillation over Southeastern China

A persistent heavy rainfall (PHR) process usually undergoes a suppressed phase, a developing phase, an active phase, a decaying phase, and an ending phase. This study tries to determine the PHR phase related to 10-30-day (high-frequency) intraseasonal oscillation (HF-ISO) over Southeastern China (including South China (SC) and the Yangtze River Basin (YRB)), by using the daily precipitation from the China Meteorological Administration (CMA) surface stations in China. We found that the HF-ISO associated with the PHR has a larger amplitude in the positive phase relative to that in the negative phase. Considering the asymmetry of the PHR process, we divide the PHR process into 5 phases with 1std in the positive phase and - 0.75std in the negative one as the threshold, that is, the suppressed phase, developing phase, active phase, decaying phase, and ending phase. During the different phases, the variables are in a wave-train-like pattern and strike from southeast to northwest. In addition to water vapor, convective instability (CI), and moisture static energy (MSE), the anomalies of other variables are not evident in the developing phase and decaying phase. Furthermore, the extremes of moisture, CI, and MSE are found ahead of the peak precipitation. In this study, we found that the moisture tendency is much larger in the suppressed phase than in the ending phase over both SC and the YRB. Thus, in the following phase, more (less) moisture leads to a stronger (weaker) convective instability and a more (less) recharge of MSE, and favoring (unfavoring) for the development of PHR. Over SC, the main reason for the difference of the moisture tendency during the suppressed phase and the ending phase is the meridional advection based on the mean state and eddy-eddy flow interactions. Over the YRB, the difference of the moisture tendency is due to the advection by the anomalous zonal wind across the mean moisture gradient.